Basic concepts (mod 4 linked)

Question 1

What is Enthalpy? State the SI unit for enthalpy and its symbol

Answer 1

Enthalpy is the measure of potential energy stored in chemical bonds.

The SI unit for enthalpy is KILOJOULES PER MOLET.

(Chemistry uses kilojoules as the norm, therefore be careful when converting values in the Q = MCΔT)

The symbol for Enthalpy is H and therefore when finding the change in enthalpy, it is expressed through ΔH

Question 2

What does the universe favor for enthalpy? What is the natural tendency?

Answer 2

The universe favours minimum enthalpy, meaning that it tends to favour less energy present.

Thus, the universe favours exothermic reactions, as energy is released in the forward direction towards the product side and therefore, less energy is available afterwards.

There is a natural tendency for enthalpy to decrease (Don’t have to explicitly describe the direction forward or reverse for this one)

Question 3

In an exothermic reaction, which side of the reaction is favored? The product or reactant side? Why?

Answer 3

The product side is favored, since exothermic reactions release energy. This means that the product side of the equation would have a minimum enthalpy. The universe favors whichever side has minimum enthalpy and in an exothermic reaction, it is evidently the product.

Therefore, the product side of an exothermic reaction is favored

Question 4

Which side of the chemical equation is favoured by the universe in an endothermic reaction? Why?

Answer 4

In an endothermic reaction, the reactant side is favoured. This is because energy is absorbed, meaning that energy is lower on the reactant side and greater on the product side. The product side is the final form of the initial, meaning that it is the initial reactant side with additional energy. Therefore, the universe favours the side with minimum enthalpy, which is less energy.

The reactant side has less energy compared to the product side in an endothermic chemical equation.

Therefore the reactant side is favoured.

Question 5

What is entropy? What are the SI units and symbols for entropy? What is the order of entropy increase regarding the states of matter?

Answer 5

Entropy is the measure of disorder and chaos within a system. A change in entropy is expressed through ΔS and the SI unit for entropy is Kilojoules per Kelvin.

The order of entropy increase is in the following order:

Solids –> Liquids –> Aqueous –> Gases

Question 6

What does the universe favour for entropy?

Answer 6

The universe favours maximum entropy.

This also means that the universe favours an increase in entropy, meaning there is a tendency for increased chaos and disorder/randomness.

Question 7

If both sides of a chemical equation have gases, then how can you tell the direction of entropy and if it is increasing or decreasing?

Answer 7

Entropy is increasing or at its maximum on the side where there is a greater number of moles of gas molecules.

Therefore, don’t count the amount of particles, rather the amount of moles on each side and see which one has more.

Greater number of moles of gaseous particles mean entropy is at its maximum and thus increasing.

Question 8

Under what circumstances would no chemical reactions occur? (Hint, reference and explain using natural tendencies and what the universe favours)

Answer 8

The universe favours minimum enthalpy and maximum entropy.

As such, if a reaction had minimum enthalpy on the reactant side and had maximum entropy also in the reactant side, then no reaction will ever occur. This is because the reactant side has already achieved what the universe wants.

Therefore, there will be no reaction if the entropy is at its maximum and enthalpy at its minimum - both on the reactant side of the chemical equation.

Question 9

When will a reaction proceed until completion?

Answer 9

When the reactant side of a chemical equation does not have either of the universal tendencies, then the reaction will do whatever it takes in order to fall into the favour of the universe.

Therefore, in order to reach maximum entropy and minimum enthalpy (which the universe favours) it must have minimum entropy and maximum enthalpy on the reactant side

Therefore, the product side would have to favour the universe, meaning that there is maximum entropy and minimum enthalpy in the product side.

• Minimum enthalpy and maximum entropy favours the products of the chemical reaction

Question 10

What does the ‘heat term’ or ‘energy term’ imply? (E.g. +46 kJ in a chemical equation)

Answer 10

The heat term or energy term simply means energy

However, whichever side the heat term is in, that side holds minimum enthalpy.

Therefore, if the symbol/value was on the reactant side (left hand side), the reactants would have minimum enthalpy, thus making it an endothermic reaction

But if the heat term was on the product side, this side would now have minimum enthalpy and thus, the reaction favours a decreasing enthalpy in the forward direction.

Question 11

What do we call the two main directions, when talking about what is happening to the entropy or enthalpy in a chemical equation?

Answer 11

Entropy and Enthalpy is either increasing or decreasing in the forward direction or the reverse direction.

Therefore, ‘Forward’ or ‘Reverse’

Question 12

When is a reaction exothermic? Refer to enthalpy

Answer 12

A reaction is exothermic when the enthalpy is less than zero.

ΔH < 0

(This means energy has been released into the atmosphere in the form of different types of energy, specifically heat)

Question 13

When is a reaction endothermic, with reference to enthalpy

Answer 13

When ΔH > 0

Since energy has been absorbed into the system

Question 14

what happens when entropy is greater than zero? ΔS > 0

Answer 14

If ΔS > 0, the system is getting more chaotic

Question 15

What happens when entropy is less than zero? ΔS < 0

Answer 15

If ΔS < 0, then entropy is decreasing within the system

Question 16

What is Gibb’s Free Energy?

Answer 16

Gibb’s Free Energy is defined as the amount of energy available to do work. It also measures whether a reaction is spontaneous or non-spontaneous

Question 17

What is a spontaneous reaction? What does it mean to be thermodynamically favoured?

Answer 17

A spontaneous reaction is a reaction that occurs naturally. Such examples include the car rusting or an ice melting.

It just happens by the rules and laws of nature.

A spontaneous reaction is basically saying the same thing as being thermodynamically favoured. Therefore, it is the same thing

Question 18

What is a non-spontaneous reaction?

Answer 18

A non-spontaneous reaction is a reaction where there must be a constant input of energy to initiate or keep going a chemical reaction. It is not natural and thus requires an external force to contribute and thus produce a reaction.

An example of a non-spontaneous reaction is photosynthesis, which requires sunlight or light energy in order to start the production process of making sugar.

Question 19

What does it mean if ΔG > 0?
What about when ΔG < 0
ΔG = 0?

Answer 19

When ΔG > 0, it is spontaneous

When ΔG < 0, it is non-spontaneous

When ΔG = 0, it is at equilibrium

Question 20

A reaction will be spontaneous when? (Refer to Gibb’s Free Energy)

Answer 20

A chemical reaction will be spontaneous meaning that it will occur naturally if it aligns with the universal tendencies of maximum entropy and minimum enthalpy. Thus, a reaction will be GUARANTEED spontaneous when ΔG > 0

Question 21

What does a ΔG^0 (degree symbol) mean?

Answer 21

This symbol stands for the change in Gibb’s Free Energy when substances are in their standard states, such as their standard temperatures or atmospheric pressure. (A gas to be 1 atm and a solution to be one molar concentration) moles per litre

Question 22

What is the formula for ΔH?

(hint: think of combustion)

Answer 22

First of all Q = mcΔT

Secondly,

ΔH = - Q / n

ΔH = ‘energy released or heat energy’ over the number of moles of that substance